Study on the movement measurement for an objective lens actuator in optical pickup

Abstract

This paper deals with the actuator movement measurement for objective lens actuator in optical pickup. An optical pickup is an optical apparatus to record and/or reproduce information from an optical disc, and move in a radial direction on the optical disc. The objective lens actuator helps move an objective lens in a tracking direction, a focusing direction, and/or a tilting direction, spot the laser beam on a correct portion of the optical disc. It basically carries out tracking and focusing movements, i.e., a biaxial movement. For high recording density media, the optical pickup actuator requires a triaxial or quadriaxial movement including a tilting movement in addition to the existing biaxial movement. For information recording and/or reproducing correctly, the movement range of the actuator should be proper to accurately follow the disc deviation. Alternative methods of movement measurement have been proposed. One method employed now uses a plane plate reflecting laser beam which describes the movement. It requires two laser devices for tracking direction and the focusing direction measurement respectively. Here we propose a method for describing the tracking and focusing movements on one laser device. In this method an external cone is used, with the same weight as the objective lens. It could reflect laser beam back to the linear CCD. The diameter of the external cone base is equal to that of the objective, which is crucial for a stable placement. The ratio of height to width of the external cone is designed and optimized, so that to satisfy the accuracy of the movement measurement and minimize the device. Measurement with this method has proved satisfactory for certain CD/DVD optical pickup.